1. Field of the Invention
The present invention relates generally to data networks, and more specifically to a technique for determining the signal-to-noise ratio value on one or more channels of an access network.
2. Background
Broadband access technologies such as cable, fiber optic, and wireless have made rapid progress in recent years. Recently there has been a convergence of voice and data networks which is due in part to US deregulation of the telecommunications industry. In order to stay competitive, companies offering broadband access technologies need to support voice, video, and other high-bandwidth applications over their local access networks. For networks that use a shared access medium to communicate between subscribers and the service provider (e.g., cable networks, wireless networks, etc.), providing reliable high-quality voice/video communication over such networks is not an easy task.
One type of broadband access technology relates to cable modem networks. A cable modem network or xe2x80x9ccable plantxe2x80x9d employs cable modems, which are an improvement of conventional PC data modems and provide high speed connectivity. Cable modems are therefore instrumental in transforming the cable system into a full service provider of video, voice and data telecommunications services. Digital data on upstream and downstream channels of the cable network is carried over radio frequency (xe2x80x9cRFxe2x80x9d) carrier signals. Cable modems convert digital data to a modulated RF signal for upstream transmission and convert downstream RF signal to digital form. The conversion is done at a subscriber""s facility. At a Cable Modem Termination System (xe2x80x9cCMTSxe2x80x9d), located at a Head End of the cable network, the conversions are reversed. The CMTS converts downstream digital data to a modulated RF signal, which is carried over the fiber and coaxial lines to the subscriber premises. The cable modem then demodulates the RF signal and feeds the digital data to a computer. On the return path, the digital data is fed to the cable modem (from an associated PC for example), which converts it to a modulated RF signal. Once the CMTS receives the upstream RF signal, it demodulates it and transmits the digital data to an external source.
FIG. 1 is a block diagram of a typical two-way hybrid fiber-coaxial (HFC) cable network system. It shows a Head End 102 (essentially a distribution hub) which can typically service about 40,000 homes. Head End 102 contains a CMTS 104 that is needed when transmitting and receiving data using cable modems. Primary functions of the CMTS include (1) receiving baseband data inputs from external sources 100 and converting the data for transmission over the cable plant (e.g., converting Ethernet or ATM baseband data to data suitable for transmission over the cable system); (2) providing appropriate Media Access Control (MAC) level packet headers for data received by the cable system, and (3) modulating and demodulating the data to and from the cable system.
Head End 102 connects through pairs of fiber optic lines 106 (one line for each direction) to a series of fiber nodes 108. Each Head End can support normally up to 80 fiber nodes. Pre-HFC cable systems used coaxial cables and conventional distribution nodes. Since a single coaxial cable was capable of transmitting data in both directions, one coaxial cable ran between the Head End and each distribution node. In addition, because cable modems were not used, the Head End of pre-HFC cable systems did not contain a CMTS. Returning to FIG. 1, each of the fiber nodes 108 is connected by a coaxial cable 110 to two-way amplifiers or duplex filters 112, which permit certain frequencies to go in one direction and other frequencies to go in the opposite direction (different frequency ranges are used for upstream and downstream paths). Each fiber node 108 can normally service up to 2000 subscribers. Fiber node 108, coaxial cable 110, two-way amplifiers 112, plus distribution amplifiers 114 along with trunk line 116, and subscriber taps, i.e. branch lines 118, make up the coaxial distribution system of an HFC system. Subscriber tap 118 is connected to a cable modem 120. Cable modem 120 is, in turn, connected to a network device 122, such as a subscriber computer.
In order for data to be able to be transmitted effectively over a wide area network such as HFC or other broadband computer networks, a common standard for data transmission is typically adopted by network providers. A commonly used and well known standard for transmission of data or other information over HFC networks is the Data Over Cable System Interface Specification (DOCSIS). The DOCSIS standard has been publicly presented by Cable Television Laboratories, Inc. (Louisville, Colo.), in a document entitled, DOCSIS 1.1 RF Interface Specification (document control number SP-RFIv1.1-I04-000407, Apr. 7, 2000). That document is incorporated herein by reference in its entirety for all purposes.
In conventional DOCSIS systems, the CMTS may include a plurality of physically distinct line cards having appropriate hardware for communicating with cable modems in the network. Each line card is typically assigned to a separate DOCSIS domain, which is a collection of downstream and upstream channels for which a single MAC Allocation and Management protocol operates. Typically, each DOCSIS domain includes a single downstream channel and one or more upstream channels. The downstream channel is used by the CMTS to broadcast data to all cable modems (CMs) within that particular domain. Only the CMTS may transmit data on the downstream. In order to allow the cable modems of a particular DOCSIS domain to transmit data to the CMTS, the cable modems share one or more upstream channels within that domain.
Channel Quality Detection
It will be appreciated that the performance of data communication in a conventional cable network may be dependent upon channel conditions of the upstream and/or downstream channels in the cable network. For this reason, it is desirable to monitor conditions of the upstream channels of the cable network in order to provide for effective management and use of each of the upstream channels. For example, if it is determined that conditions on a particular upstream channel are below acceptable quality levels, the CMTS may instruct cable modems on that channel to hop to a second upstream channel and begin using the second upstream channel for communicating with the CMTS.
One indicator which is typically used to evaluate channel conditions of a selected channel is the signal-to-noise ratio (SNR) associated with that channel. Typically, in most HFC networks, the SNR value for each upstream channel is measured and calculated using an off-the-shelf component such as, for example, the ASIC chip BCM3137, manufactured by Broadcom Corporation of Irvine, Calif.
Conventional techniques for determining the SNR value of a selected upstream channel are typically based upon Error Vector Magnitude (EVM) calculations, which are generally known to one having ordinary skill in the art. For example, a conventional ASIC chip configured to measure SNR on a particular upstream channel of an HFC network continuously monitors the upstream channel for received signals. When a signal is detected, the ASIC analyzes the signal to determine codes or symbols which may be embedded therein according to a predetermined format or protocol. For each symbol detected in the received signal, the ASIC determines the Error Vector Magnitude (EVM) for that symbol. The EVM value represents the amount of deviation that exists between the vector position of the detected symbol and the theoretically ideal position of that symbol. Once the EVM value for a particular symbol has been determined, a SNR value may then be calculated using a predetermined formula generally known to one having ordinary skill in the art. Additionally, it is typically the case that the ASIC computes an average SNR value for the selected upstream channel using the EVM values associated with that channel. A more detailed description of channel SNR measurement is provided in U.S. Pat. No. 5,862,451, to Grau, et al., issued on Jan. 19, 1999, and entitled xe2x80x9cChannel Quality Management in a Cable Telephony Systemxe2x80x9d, which is incorporated herein by reference in its entirety for all purposes.
Using conventional EVM-based techniques, the calculation of the SNR value for a selected channel may only be performed if a signal lock is achieved on the selected channel. Thus, under ideal network conditions, the EVM-based technique for determining SNR values for selected upstream channels may yield satisfactory results. However, as conditions on a particular upstream channel deteriorate, the calculated SNR value for that channel may include a substantial amount of error, making the calculated SNR value substantially inaccurate. Additionally, the calculated SNR value does not take into account corrupted data received on the selected channel, which further compromises the accuracy of the SNR calculation. Further, in situations where only a relatively small amount of data is available to be used for calculating the SNR value (such as, for example, periods immediately following initialization or restart of a particular channel), the accuracy of the calculated SNR value (using conventional techniques) may be extremely poor. Accordingly, it will be appreciated that erroneous or inaccurate calculation of SNR values may result in poor management and performance of channels in the data network.
A further limitation of conventional EVM-based SNR calculating techniques is that such techniques are incapable of performing SNR analysis on a node by node basis. For example, the ASIC chip BCM3137 is incapable of performing SNR analysis for one or more specific modems of an upstream channel. Moreover, the ASIC is incapable of distinguishing data received from different cable modems on the selected channel. As a result, it is not possible, using conventional SNR calculating techniques, to determine whether a relatively low SNR value is attributable, for example, to ingress noise on the selected channel or is attributable to one or more faulty cable modems on the selected channel.
In light of the above, it is desirable to provide an improved SNR calculation technique which provides greater flexibility and accuracy.
According to a specific embodiment of the present invention, a method and computer program product are disclosed for determining a carrier-to-noise ratio (CNR) value for a selected channel of an access network. The access network includes a plurality of network nodes which communicate with a Head End via the selected channel. A background noise level on the selected channel is measured during a first time interval during which none of the network nodes are transmitting signals on the selected channel. A first carrier signal strength on the selected channel is also measured during a second time interval during which a first network node is transmitting at least one signal on the selected channel. A CNR value for the selected channel may then be calculated using the measured background noise level information and the measured first carrier signal strength information. According to a specific implementation, the calculation of the CNR value for the selected channel is not based upon an Error Vector Magnitude value associated with the signal transmitted from the first network node.
According to an alternate embodiment of the present invention, a method is disclosed for determining a signal-to-noise ratio (SNR) value for a selected channel of an access network. The access network includes a plurality of network nodes which communicate with a Head End via the selected channel. A background noise level on the selected channel is measured during a first time interval during which none of the network nodes are transmitting signals on the selected channel. A first signal strength on the selected channel is also measured during a second time interval during which a first network node is transmitting at least one signal on the selected channel. An SNR value for the selected channel may then be calculated using the measured background noise level information and the measured first signal strength information. According to a specific implementation, the calculation of the SNR value for the selected channel is not based upon an Error Vector Magnitude value associated with the signal transmitted from the first network node.
Another embodiment of the present invention is directed to a system for determining a CNR value for a selected channel of an access network. The access network includes a plurality of network nodes. The system comprises a Head End in communication with at least a portion of the network nodes. The Head End comprises at least one CPU, memory, at least one interface configured to receive signals from the network nodes via the selected channel, and at least one analog-to-digital converter configured to analyze signals on the selected channel. The Head End is configured to measure a background noise level on the selected channel during a first time interval when none of the network nodes are transmitting signals on the selected channel. According to a specific embodiment, the Head End may configured to instruct at least a portion of the network nodes not to transmit any signals on the selected channel during the first time interval. The Head End is also configured to measure a first signal strength of a first signal transmitted on the selected channel by a first network node during a second time interval. According to a specific implementation, the Head End may be configured to instruct the network nodes to allow only the first network node to transmit signals on the selected channel during the second time interval. Additionally, the Head End is also configured to calculate a CNR value for the selected channel using the measured background noise level information and the measured first signal strength information.
Additional objects, features and advantages of the various aspects of the present invention will become apparent from the following description of its preferred embodiments, which description should be taken in conjunction with the accompanying drawings.